This invention relates to a polygon circuit used for input signal level compression in a signal processing circuit in, for example, a color television camera or the like.
FIG. 1 is a diagram showing a prior art polygon circuit. In the FIG. Q21 to Q28 denote transistors, and R21 to R32 denote resistors. Q23 and R24; Q24 and R25; and Q27 and R26 respectively form current sources permitting a constant current to flow in accordance with a current source current setting voltage Vb applied to an input terminal 23. To simplify the explanation, it is assumed that R24, R25 and R26 have the same value, and the same collector current I0 flows through Q23, Q24 and Q27. 20 denotes an input terminal for an input signal Vin, 21 denotes an input terminal for a reference signal Vref, 22 denotes an input terminal for a break-point setting voltage Va, 23 denotes an input terminal for a current source current setting voltage Vb, 24 denotes an output terminal for an output signal Vout, and Vcc denotes a power supply voltage.
Now the operation will be described.
First, it is assumed that the collector voltage VCQ22 satisfies VCQ22&lt;Va.
In this case, if Q26 operates as an emitter-follower. If the base-emitter voltage of Q26 is VBEQ26, the base voltage VEQ28 of Q28 is given by EQU VBQ28=Va-VBEQ26 (1)
The emitter voltage VEQ28 of Q28 satisfies EQU VEQ28&lt;VCQ22 (2)
The base-emitter voltage VBEQ28 of Q28 is thus small from VCQ22&lt;Va, and (1) and (2), and Q28 is turned off, and the output signal Vout is given by: ##EQU1##
Now it is assumed that VCQ22&gt;Va.
In this case, Q28 operates as an emitter follower, and the output signal Vout is given by: ##EQU2##
FIG. 2 is a diagram showing the input/output characteristics of the prior art polygon circuit. The gradient G in the region VCQ22&lt;Va, i.e., in the region ##EQU3## is given by: EQU R22/R21
The gradient G of the input/output characteristics in the region of VCQ22&gt;Va, i.e., in the region ##EQU4## is given by: ##EQU5##
The polygon circuit is mainly implemented in a monolithic integrated circuit, and the break-point setting voltage Va is obtained by voltage-dividing the power supply voltage Vcc by use of resistors R29 and R30 disposed outside of the integrated circuit. The reference voltage Vref is a reference level for the input signal. Where a video signal is used as an input signal, the black level is input being clamped to Vref.
Vcc-I0 R22 is the reference level for the output signal Vout.
As the prior art polygon circuit is constructed as described above, when the characteristics of R22, Q24 and R25 vary, the break-point set by R29 and R30 fluctuates with respect to the reference level Vref for the input signal or with respect to the reference level Vcc-I0 R22 for the output signal Vout.